Synovial membrane histopathology. Images representative of the synovial membrane inflammation one week (A-C) and six weeks (D-F) after injection of saline ( A , D ), 250 U (B, E) or 500 U of collagenase ( C , F ). An increased number of synovial lining cell layers, proliferation of subsynovial tissue and infiltration of inflammatory cells are observed. Changes were most pronounced with the 500 U dose, one week after injection, receding thereafter. Bar: 50 μm.